A flat roof can function well as along as it's built properly. Some considerations must be taken. This eHow can help you in its construction.
Span the gap between two walls with joists. Figure out the "maximum span" of the joists depending on the span and the load that the roof might have to hold when you build a flat roof.
Cover the joists with 5/8 plywood or a material of your choice. Allow for a 1/8-inch gap at all of the joints to allow the wood to expand and contract.
Insert "noggins" between the joists as you build a flat roof. These noggins help to prevent the joists from twisting.
Clean the plywood before bringing up the rubber for the flat roof. Remove any debris that could potentially tear the rubber and weaken its waterproof properties.
Cut pieces of rubber at the lengths needed for the flat roof you are building plus 9 inches all the way around extra.
Overlap the sections of rubber as you lay them down and glue the seams when you are finished laying down all of the rubber sheets. If you have to fit the rubber over any pipes, cut a hole a little larger than the pipe and slip the rubber over the pipe. Make a tight seal around the pipe.